Process for producing hard board



Oct. 6, 1959 J. G. MElLx-:R ET AL 2,907,071

RRocEss RoR PRonucING HARD BOARD Filed Aug. 11, 1955 N TW Smm .m s cfm w n mmm Em wen 4M o N A W .n INI. .A

United States Patent PROCESS FOR PRODUCING HARD BOARD John G. Meiler, Tacoma, and Earl A. N. Johnson, Shelton, Wash., assignors to Rayonier Incorporated, Shelton, Wash., a corporation of Delaware Application August 11, 1955, Serial No. 527,875

11 Claims. (Cl. 18-47.S)

This invention relates to hardboard formed of compressed Wood bers bonded together with a thermoset resin, and has Afor its object the provision of an improved process of making such hardboard. The process of the invention comprises a combination of conditions and operations for converting loose bulky -wood bers, such as result from wood grinding, into an improved dense hardboard in an eicient practical operation with relatively inexpensive machinery. The hardboard produced in accordance with the invention is formed as a sheet or web of continuous or indefinite length of high quality and density and is relatively thin, including sheets or webs of considerable flexibility.

The process of the invention comprises three stages of operation, namely (1) preliminary treatment of wood bers such as are used in wood pulp with resin and preferably also with wax and the formation of a thick mat, (2) compression of the mat momentarily under high pressure and elevated temperature, and (3) nal compression under low pressure and high temperature for a time sufficient to cure the resin.

In the continuous embodiment of this invention, a loose bulky mass of wood ber containing moisture is coated with a thermosetting resin and wax and is compacted into a low density mat of uniform thickness. The mat is heated and carried by a belt or belts through the nip of heated pressure rolls and nally cured under a relatively vlow pressure.

The application of heat is a critical factor in the process. It is necessary to bring the temperature of the mass to a point where the wood ber becomes somewhat plastic and easily molded during the application of high pressure. Since the high pressure is applied only momentarily as the -web passes between the rolls, it is necessary to apply some heat before the web enters the rolls in order that the temperature be high enough to permit adequate compression. It is also important that moisture be present during the application of high pressure since it serves as a plasticizer for the wood. The time and temperature of any preheating is, however, limited by the necessity of avoiding curing the resin and losing moisture before the application of high pressure. Moisture loss can be avoided by holding the web under pressure during the preheating stage.

The preliminary compression of the bers may be carried out with belts and rolls in an integrated part of a continuous operation to a low density, for example to about 0.15 sp.gr. In the high pressure compression stage, the mat is fed between rolls at temperatures which make the bers soft and somewhat plastic, such as are used in bending wood, and under a pressure which is high enough to achieve nal thickness but not to injure the bers, and without appreciable loss in moisture or setting of the resin.

One of the important aspects of the invention is the feeding of a continuous mat of wood bers containingthe required amount of moisture, a thermosetting resin and a wax, between driven rolls with such clearance that the mat is compressed, preferably to less than final thick- 2,907,071 Patented Oct. 6, 1959 ICC be carried out by compressing the mat in direct contactl with opposed pressure rolls, or between belts which are pressed between pressure rolls, the term press temperature" refers to the temperature of the contacting belt or rolls as the case may be. t

1n the final or low pressure stage, the web is heated to a temperature that will set the resin, in the range of press temperatures of to 230 C. for from 5 to 90 seconds and under a pressure which is merely enough to attain the desired final thickness, for example at a pressure between 50 and 300 p.s.i. The low pressure stage of the operation results in marked economies in plant cost and in production cost since the operation may be carried out in a belt press or in a low pressure at bed press of the type used to compress plywood. The invention eliminates the extremely heavy and costly at bed presses now used to form hardboard, and also permits the operation to be carried out continuously. The invention provides a practical process of making a superior product at an increased' speed on much simpler and less expensive equipment than is presently being used.

The hardboadof the invention may be formed of the wood of different. species of trees, the coniferoustrees such as cedar, pine, hemlock and Douglas/r being suitable. The logs may be barked and cut into chips such as are used in forming pulp. However, since some bark is not objectionable in some' nal products-the barking may be partly or entirely eliminated, depending on the particular species of tree and the proportionof bark'to wood. The logs are run through achipper as is common practice in the wood pulping industry and the chips are softened by steaming in direct contact with steam under any suitable conditions, for example, at a pressure of from 25 to 50 p.s.i. for about 4 minutes.

The softened fibers of the chips must be subdivided without excessive damage of the individual fibers. The ber separation may be accomplished effectively by passing the steamed chips through a double disc attrition mill, such as a Bauer rener in which the discs are set not closer than l0 mils so as not to damage the bers. The subdivided bers are dried gently to avoid ber damage, as with a current of warm air, to a moisture content of from 5% to 25% based on the bone dry weight of the bers. A thermosetting resin either as a solution or a powder and a hydrocarbon wax are then sprayed onto the bers to eect a uniform coating.

The resin used must be a thermosetting type that will set-up or cure completely during the last stage which is the low pressure stage. The most suitable resin which meets these requirements is a phenol formaldehyde resin. However, similar resins'such as the ureaformaldehyde and the resorcinol formaldehyde resins may also be used. The amount of resin used need be only enough to bind the bers together and impart the required strength to the board which increases generally with increases in the resin content. Thethermosetting resin, however, isl the expensive portion of the board and this limits its practical use to, say, 10%. Less than about 0.5% resin yields a weak unsatisfactory product, and about 6% is a practical upper limit of resin content.

The wax is a water repellant in the nished hardboard. While there are a number of suitable jhydrocarbon waxes it is advantageous to use petrolatum because of its low price and overall suitability. Some wax is important for the foregoing purposes, but since it contributes nothing to the drystrength of the finished board, it should 4be used in amounts varying from 0.5 to 3%, usually not over 2% based on the weight of the bers.

Theaccompanying drawing illustrates diagrammatically by ow sheet an arrangement of apparatus for carrying out .a .process of the invention.

In carrying out a complete operation of the invention, moisture-containing bers coated with the thermosetting resin and wax are formed into a continuous mat of uniform density by any suitable method `commonly used in this art. For example, the mat may be prepared by showering the bers onto a moving belt and shaving the resulting layer to ka uniform thickness of say 5 inches with a rotary brush, depending on the thickness desired in the nal hardboard. This very loose continuous mat is then compacted as by passing it between belts supported on rolls to a thickness of about l inch and a sp. gr. of about 0.10. The compacted continuous mat of wood bers .having the desired moisture content, varying from 3.5 to 25%, preferably from 5% to 15%, a thermosetting resin and petrolatum wax is continuously preheated and passed to the high pressure stage.

The second or high-pressure stage of the invention preferably comprises running the compressed mat of bers between high-pressure rolls at a pressure of from 1000 to 5000 p.l.i. (pounds per linear inch) at a speed of from 25 to 100 feet per minute at a press temperature of from 100 to 230 C., usually in the range of 150 to 200 C.

The high-pressure, temperature and moisture condi-A tions used in this high-pressure operation are for the purpose of causing `the so-called plastic fiow of the bers to take place without setting up or curing the resin to any appreciable extent, and compressing the bers quickly and continuously to the required density or thickness. As a result ofthe peculiar properties thus imparted to the sheet, a dense sheet is obtained which can be cured into a uniform, hard, strong, high density product at .the relatively low pressures employed in the nal stage in which the board is iinishedunder relatively high temperature and low pressure. The board is preferably cornpressed to a thickness not greater than that of the final board and to a sp. gr. of from 0.90 to 1.4 and preferably over 1.1.

The time, temperature and pressure in the high pressure stage are inter-related. The objective is to regulate these conditions to impart plastic ow to the fibers without injuring them and without appreciable curing of the resin, in the case of thermosetting resins, beyond a tacky state. By passing the web with a moisture content of from 5 to 25% between the rolls at a speed of from 25 to 100 feet per minute, at a press temperature of from 100 to 230 C., preferably from 150 to 200 C., and at a pressure of from 1000 to 5000 p.l.-i. preferably 1000 to 2500 p.l.i., the resulting web has high density without appreciable if any, ber damage and without completely setting the thermosetting resin. This part ofthe operation makes possible a continuous integrated operation and results in a sheet of indefinite length that can be cured at low pressures to nal density. This high-pressure stage permits the elimination of the high-pressure presses commonly used in curing hardboard.

The third and nal stage in the process of the invention is the curing of the resin component of the high density board or sehet. This is accomplished by pressing the material as it comes from the high pressure rolls at a relatively high press temperature, 150" to 230 C., and a relatively lower pressure, 50 to 300 p.s.i., for a suicient time to cure the resin.

If the compressed web or sheet from the high pressure stage is thin enough to bend over a radius of, say, about -30 inches, usually from 0.005 to 0.05 inch thick, it can 4 be run continuously through a belt press such as the machine of Letters Patent No. 2,591,995, and sold under the name Rotocure. While the machine does not apply preheat, under pressure it can be used to produce a product of vindefinite length at slow speeds. If the sheet is too thick for bending around rolls of a belt press machine, the sheet may be cut into lengths of, say, 8 and 16 feet, for compression and curing in a conventional dat bed press. It is one of the important features of the invention that the curing at nal density can be accomplished under pressures obtainable in an ordinary plywood press.

The temperature in the last stage should be high enough to cure the resin but not so high that the product will be burned, for example temperatures between 150 to 230 C. are satisfactory. The pressure during curing need only be in the range of from 50 to 300 p.s.i. The curing of the resin, of course, takes a nite length of time and varies with the thickness of the sheet. Sheets bonded with a phenol formaldehyde thermosetting resin and about 0.01 inch thick can be cured in 10 seconds or less, but a sheet about 0.050 inch thick will require about 30 seconds. If the sheet is about 0.25 inch. thick it may require 10 minutes to cure the resin. However, sheets within 0.005 to 0.1 inch thick can generally be cured in from a few seconds to 1.5 minutes.

The process is described herein as being continuous and this term comprehends and defines both the completely continuous operation of forming a continuous mat and sheet of indefinitel length and the modiiied operation in which the sheet from the high-pressure stage is cut into sections for nal curing in a at bed press, the important and characteristic feature being that the mat and sheet produced in the high-pressure stage are formed continuously.

The hardboard resulting from the process of the invention can be used as a substitute for paper board, or hard board known as Masonite or as a core for laminated articles. In the form of thinner sheets, varying say from 0.005 to- 0.05 inch, it can be used as honeycomb, furniture backs, book bindings, or corrugated board or as a smooth hard facing on plywood, insulating board, gypsum board, and the like. In fact, the hardboard can be used wherever a strong, smooth high density fiber board or sheet is used.

The following examples are illustrative operations carried outin accordance with the process of the invention:

Example I Hemlock ber was prepared by steaming chips at 25 p.s.i kfor 5 minutes and passing the hot chips through a doubled disc debrator with a clearance of 20 mils between the discs. The ber was partially dried in a hot air streamand was sprayed with 20% aqueous phenolformaldehyde solution and molten petrolatum to add 6.0% resin solids and 2.0% petrolatum. The coated ber was then dried to a moisture content of 10.6% by a stream of. warm air. Mats were prepared by blowing the treated ber onto a screen with a vacuum behind it. The felt was shaved to a thickness of 3 inches by a rotary wire brush and prepressed to a thickness of 0.5 inch. The mat was then fed into a Rotocure with a curing drum 72 inches in diameter with an estimated nip pressure of 2500 p.l.i. and a belt temperature of C. It is estimated that the rnat was preheated to 60-100 C. as it passed over the pressure roll before entering the nip. The belt pressure against the curing drum was 47 p.s.i., the temperature was 175 C. and the retention time was 2 minutes terminated by a second pass kthrough the pressure rolls. The nished sheet had a thickness of 0.1 inch, a specic gravity of 1.03 and a modulus of rupture of 5400 p.s.i.

Example II Hemlock ber was prepared by the procedure described `in Example No. I. The partially dried ber was sprayed with 20% aqueous phenol-formaldehyde Iresin and molten petrolatum to add 6% and 2% solids respectively. The coated ber was then dried to a moisture content of 10% by a streamof warm air. Matsvwere prepared by blowing the treated ber onto a screen with a vacuum behind it. The belt was shaved to a thickness of l inch by means of a rotating wire brush.

The mat was passed between `steel rolls 22 inches in diameter with a pressure of 2500 p.l.i. and a linear v elocity of 28.8 ft./min. at a temperature of 100 C. The compressed mat was then cured in a at bed press at 150 p.s.i. and a platen temperature of 180 C. for l minute. The nished sheet had a thickness of 0.035 inch, a specicgravity of 1.07 and a tensile strength of 4500 p.s.i.

Example III Hemlock ber, prepared by the procedure described in Example No. I, was sprayed with 20% aqueous phenolformaldehyde solution and molten petrolatum to add 4% and 2%Y solids respectively. The coated ber was then dried to a moisture content of 10% by a stream of warm air. Mats were prepared as in' Example No. II, and shaved to a thickness of 1 inch. p

The mat was passed between steel rolls 22 inches in diameter with a pressure of 2500 p.l.i. and a linear velocity of 28.8 ft./min. at a press temperature of 120 C. The compressed mat was then cured in a at bed press at 300 p.s.i. and a platen temperature of 200 C. for 15 seconds. The finished sheet had a thickness of 0.037 inch, a specic gravity of 1.12 and a tensile strength of 4900 p.s.i.

Example IV Western red cedar ber, prepared by the procedure described in Example No. I, was sprayed with 20% aqueous solution of a bark extract adhesive as described in U.S. patent application Serial No. 367,364, led July l0, 1953, Example 7, and molten petrolatum to add 6% and 2% solids respectively. The coated ber was then dried t a moisture content of 11.7% by a stream of warm air. Mats were prepared as in Example No. II and shaved to a thickness of 0.75 inch.

The mat was pressed three times between steel rolls 22 inches in diameter with a pressure of 5000 p.l.i. and a linear velocity of 28.8 ft./min. The temperature of the mat was between 60 and 100 C. The compressed mat was then cured in a at bed press at 60 p.s.i. and a platen temperature of 180 C. for 1 minute. The nished sheet had a thickness of 0.022 inch, a specic gravity of 1.14 and a tensile strength of 3200 p.s.i.

Example V A hemlock ber mat, prepared as described in Example No. II, was fed into a Rotocure belt press with a curing drum 72 inches in diameter, with an estimated nip pressure of 2500 p.l.i. and a belt temperature of 145 C. The linear velocity was 4 t./min. The belt pressure against the curing drum was 47 p.s.i., the temperature was 175 C., and the retention time between belt and drum was 3.5 minutes. The nished sheet had a thickness of 0.037 inch, a specic gravity of 1.0-1 and a tensile strength of 3300 p.s.i.

We claim:

l. The process of producing hardboard sheets varying from 0.005 to 0.100 inch thick and of indenite length which comprises forming a relatively thick bulky mat 0f wood bers containing water and a thermosetting resin, preheating the bers and while they are at a temperature which softens the wood applying a pressure of at least 1000 p.l.i. momentarily and compressing the bers into a sheet having a thickness equal to or less than that of the nal board, and then in another operation compressing the sheet while at a press temperature of from 150 to 230 C. and under a pressure up to 300 p.s.i. for 5 to 90 seconds to set the resin and form a hardboard having a sp. gr. of from 0.90 to 1.3.

2. The process of producing hardboard sheets varying from 0.005 to 0.100 inch thick and having Ifrom 0.9 to 1.3 sp. gr. and of indenite length which comprises forming a relatively thick `bulky mat of wood ber containing suicient moisture and thermosetting resin to plasticize and, bind the wood ber when under heat and pressure, subjecting the mat to suicient heat to soften the bers and pressing it between rolls under a pressure above 1000 p.l.i. heated to a temperature of from 100 to 230" C. until the resulting sheet is compressed to less than 0.100inch thick, aud then curing the sheet by pressing it between opposing surfaces at pressures of from 50 to 300 p.s.i. and temperatures of from 150 to 230 C. for from 5 to 60 seconds.

3. In the process ofv claim 2, forming a mat of wood bers containing from 3.5 to 25% of moisture which have been softened by steaming and then at least partially dried prior to pressing between the rolls.

4. The process of forming hardboard sheets which comprises softening wood bers by heating them in a moist condition, mixing a thermosetting resin and wax therewith and forming said softening bers into a relatively thick mat, passing the mat at a moisture content of from 5 -to 15% between pressure rolls at the rate of from 25 to 100 feet per minute and at a pressure of from 1000 to 5000 p.l.i. to compress the mat into a web to approximately nal thickness but Without curing the resin appreciably or appreciable loss of water, then subjecting the web to nal pressing at a substantially lower pressure and at a temperature which will cure the resin until a hardboard less than 0.100 inch thick and having a sp. gr. of from 0.9 to 1.3 is obtained.

5. The process of forming hardboard sheets which comprises softening wood bers by heating them in a moist condition, mixing with the bers a thermosetting resin and a Wax, forming the bers continuously into a relatively thick mat, passing the mat at a moisture content of from 5 to 15 between pressure rolls heated to a temperature of from to 230 C. at the rate of from 25 to 100 feet per minute and at a pressure of from 1000 to 5000 p.l.i. to compress the mat into a sheet at approximately nal thickness but without curing the resin appreciably and without appreciable loss of water, then passing the sheet continuously between pressure applying belts which compress it at a substantially lower pressure and heat it at a temperature that will cure the resin until a hardboard of indenite length and nal thickness less than 0.100 inch and having a sp. gr. of from 0.9 to 1.3 is obtained.

6. The process of forming hardboard sheets which comprises softening wood bers by heating them in a moist condition, mixing with the bers a thermosetting resin and a wax, forming the bers continuously into a relatively thick mat, passing the mat at a moisture content of from 5 to 15 between pressure rolls heated tol a temperature of from 100 to 230 C. at the rate of from 1000 to 5000 p.l.i. to compress the mat into a sheet at approximately its nal thickness but without curing the resin appreciably and without appreciable loss of water, cutting the sheet into sections, compressing them at a pressure below about 300 p.s.i. in a flat bed press at a temperature that will cure the resin until a hardboard less than 0.100 inch thick and having a sp. gr. of from 0.9 to 1.3 is obtained.

7. The process of producing hardboard from loose wood bers which comprises compressing a layer of wood bers containing from 3.5 to 25 water and not more than 10% of a thermosetting resin, both based on the bone-dry weight of the bers, to form a mat of indenite length, heating. said mat suiciently to soften the bers and passing it continuously between press rolls-heated to a temperature of from 100 to 230 C. at a pressure vi between 1000 and 2500 p.1.i thereby compressing said mat into a sheet with a thickness not greater than the nai Vthickness, without appreciable evaporation of the water or setting of the resin, then passing the sheet thus formed to another compression operation and holding said sheet under a pressure of from 50 to 300 p.s.i. and at a temperature of from 150 to 230 C. to ASet the resin and produce a hardboard having a final thickness of from 0.005 to 0.100 inch and a specific gravity of from 0.9 to k1.3. Y

8. In the process of claim 7 passing the mat between the rolls while the rolls are heated to a temperature of from 150 to 200 C. at a speed of Afrom 25 to 100 feet per minute without appreciable evaporation of the moisture contained in the wood Vfibers, whereby the resulting sheet is amenable to subsequent treatment to set the resin under heat and pressure.

9. The process of forming hardboard which comprises forming a mat of indefinite iength of moist wood bers coated with a thermosetting resin and a wax, said mat having been preheated to a temperature not above 180 C. to soften the bers, passing the mat continuously between power driven rolls while at a press temperature of from 150 to 200 C. at a speed of from 25 to 100 feet per minute and compressing the mat into a sheet having at least its final thickness not exceeding 0.100

inch without appreciable evaporation 0f the wntand water or complete setting of the resi?, and then subjecting the sheet .to va pressure of from to 300 p.s.i. for from 5 to 60 seconds at a temperature of from t0 .230* C- t@ Completely ser the resin, producing. e hard 119111.11 hfwasA the desired 11121 thickness and. density the sp. gr. being from 0.9 to 1.13.

1,0. the process of claim 9 passing the sheet `from the power driven rolls directly without appreciable loss of moisture into ya machine which presses the sheet between endless belts at a pressure of from 50 to 300 p.s.i. at a press temperature of from to 230 C. in from 5 :to 60 seconds.

11. in the process of claim 9 forming a mat of wood fibers containing from 3.5 to 25% of moisture.

References Citedvin the le of this patent UNITED STATES PATENTS 

